1. Field of the Invention
This invention relates to a power circuit for an LSI (large scale integrated circuit). More particularly, it relates to a power circuit for an LSI which is able to prevent the degradation of power wirings caused by the electro-migration phenomena.
2. Description of the Prior Art
As fine processing techniques of LSIs have progressed recently, a strong need has arisen to make wirings narrower in LSIs. With this fine processing, there is a great need to make wiring layers thinner. On the other hand, the amount of consumed power per one LSI chip has increased as a result of the high integration and the enlargement of one LSI chip. According to the above-mentioned facts, the current density per cross sectional unit has increased more and more.
It is also well known that fine power wirings will be degradated and, finally, disconnected because of the electro-migration phenomena, if current above the tolerance continues to be applied to the wirings. The electro-migration phenomena is caused by the movement of metal atoms, which migrate toward the same direction of the electron flow in fine wirings. When an electric current flows in a power wiring, electrons, which flow in the reverse direction to the current flow, collide in the wiring against metal atoms, of which the wiring is made, and impart their kinetic energy to the metal atoms. As a result, the metal atoms move with electrons to cause the electro-migration phenomena. This electro-migration phenomenon is also accelarated by Joule heat generated by the current flow in the wiring. See, for example, "Prediction of the electro-migration life time of all LSI wirings", Noguti, et. al., Nikkei Micro Device, Sep. 1989, for the electro-migration phenomena.
FIG. 1 shows a typical power circuit for an LSI as an example of prior art of this invention. As shown, a plurality of circuits 3 (C.sub.1, C.sub.2, . . . C.sub.n), of which an LSI is composed, are connected between a power wiring 1 and a ground power wiring 2 of the LSI. One of the output terminals of a power supply circuit 4 is connected to a first end 5 of the power wiring 1 to supply a constant voltage V.sub.dd to each circuit of the LSI through the power wiring 1. The ground power wiring 2 is connected to the ground terminals of each LSI circuit 3. The other end 6 of the wiring 2 is also connected to ground.
In the prior art power circuit mentioned above, current I.sub.dd flows on the power wiring 1 from the end 5, which is at the side of the power supply circuit 4, towards the other end. The direction of the current flow is, thus, fixed. On the other hand, current I.sub.ss on the ground wiring 2 flows in the reverse direction to the current I.sub.dd. This direction of the current I.sub.ss is also fixed. As a result, these wirings are affected greatly by the electro-migration phenomena discussed above.
Considering the signal wirings of LSI, a signal current to charge and diacharge capacities, which are to be circuit loads, flows alternately due to the voltage change between `0` and `1`. The signal wirings can, therefore, stand strongly against the electro-migration phenomena. A constant voltage is applied to the power wirings of LSIs as mentioned above, and these wirings are affected strongly by the electro-migration phenomena.
To protect the LSI's power circuits from the electro-migration phenomena, the following three methods will be considered.
(1) To make power wiring layers thick. PA0 (2) To make power wirings wide. PA0 (3) To change the materials in power wirings.
These methods are, however, inconsistent with the basic idea for LSIs, that is, to make LSI wirings narrower and more fine.
Thus, in the prior art LSI power circuits, there has arisen a limit as to how fine LSI wirings can be made due to the electron-migration phenomena. This fact, then, prevents LSIs from being finer and more integrated.